Asynchronous Analog to Digital methods have been patented in many different forms. A flash Analog to Digital Convertor might consist of an array of parallel comparators followed by logic. Some other common techniques being used are pipeline architectures and folding architectures. This invention shows a very simple architecture which can compare and fold an analog signal in a pipeline of stages at a fast enough rate to do the conversion real time. The architecture can do this by using only the fastest transistors available on the chip. The simplicity in the signal path encourages the development of many speed enhancement methods. Applying the simplest speed enhancement methods presently allow a 4 bit Analog to Digital Convertor to follow a continuous time signal, and output every one of the 16 LSB transitions at a 1 GHz rate, while using transistors which only have a bandwidth of 4 GHz.
The fundamental folding comparator circuit is shown in FIG. 1. An almost identical circuit can be found in U.S. Pat. No. 4,069,460. This invention has discovered and exploited some important new surprises over and above the U.S. Pat. No. 4,069,460 circuit.
The first big surprise is the discovery that analog input ports can operate as digital output ports at the same time. A moving analog current is applied to the analog input node on FIG. 1. A second DC reference current is being applied to the other node. These two nodes are also being used as a single differential binary voltage output port. FIG. 2 shows how the analog input of FIG. 1 response to a full scale ramp up signal. This circuit node will toggle from one diode to two diode voltages. Not shown but at the same time the other circuit node of Dout1 is toggling from two diodes to one diode in voltage.
The second surprise is the available simplicity and speed to the analog signal path. The circuit of FIG. 1 is completely self biased by a high speed input analog signal current and a DC reference current. Since a DC reference current is constant, it is not in the signal path. Such a current source can be built using slow transistors. The input signal current only needs to be fast. This input signal current can have a high speed signal path to the output port since it is only handled by transistors contain within the folding comparator stage. Only the fastest transistors available on chip need to be used in the folding comparator's analog signal path.
The third surprise is the fact that the input current is being folded around the DC reference current before being passed on to the output port. An input ramp needs to be applied to see this. FIG. 2 shows that Ioutput1 will be in fact follow Iinput1 automatically folded around the DC level of Iref1. Then Ioutput1 can be coupled to the input of another folding comparator where the analog input signal will get folded again. This folding method is extremely simple compared to the other folding architectures.
The fourth surprise is the ease of current coupling. The circuit in FIG. 3 shows that the coupling of one folding comparator stage to another only requires a diode and another DC current source. Transistor Q1.7 is biased up with another DC current to allow signal current to be applied to the next stage in the same direction. Signal path speed is therefore not compromised because only the fastest on chip transistors need to be put in the signal path. Being able to pipeline the signal from one stage to the next using only the fastest available transistors is uncommon.
A fifth surprise was how easy it was to take a current gain of two. FIG. 5 shows how adding another transistor across the coupling diode will allow each stage to operate at the same peak to peak levels of current. This allows all transistors in the signal path to be biased up at a common optimum speed.
The sixth surprise is the extreme simplicity. The circuit of FIG. 7 is actually showing all the transistors in the signal path that are needed to implement 4 bits of high speed Level-Crossing Sampling. A Flash would require 16 much more complicated voltage comparators and also much more logic to do the same job.